1. Field of the Invention
The invention relates generally to a plasma sintering process for repair and fabrication of combustion turbine components which will provide improved results over repair techniques such as welding or wide gap brazing, for applications such as repair of cracked turbine blades or vane segment platforms; and which will allow manufacture of complex components for which casting processes are not cost effective.
2. Background Information
Utilities and other power producers have been adding capacity or replacing existing steam generators with smaller, more efficient combustion turbines. The higher efficiencies of the combustion turbines have been realized via higher operating temperatures which typically require use of gamma prime strengthened, nickel base superalloys for components such as blades and vanes. Due to the high replacement costs of these hot section components, development of new and improved repair processes is essential. Also, the advanced turbine systems typically require use of directionally solidified and single crystal hot section components. However as the design complexities and component size increase, the ability to cast single piece components in a cost effective manner decreases. In many cases, advance blade and vane design will require complex cooling geometries for which conventional casting processes are not possible.
In the past, gas tungsten arc welding has been a main primary repair process. However, the gamma prime strengthening mechanism used in manufacturing these superalloys results in hot cracking (during welding) or and/or strain age cracking (during post weld heat treatment) of the heat affected zones or of the weld metal itself. In order to alleviate this situation, repairs have been made using lower strength, solid solution strengthened filler alloys such as IN 625 (sold commercially by INCO International) for repair. Because of the poorer mechanical properties of the weld metal and high probability of microfissures in the heat affected zones, repairs have been limited to non-critically stressed regions of components. Advanced welding procedures utilizing high strength filler metals continue to result in inferior creep and fatigue properties.
In U.S. Pat. No. 5,554,837, (Goodwater et al.), laser welding a powder alloy feed, after preheating the weld area and an adjacent region to a ductile temperature, was used during manufacture and repair of jet engine components that operate at temperatures of up to 1093xc2x0 C. (2000xc2x0 F.). The problem solved there was, that as a result of such high temperature demands the components usually are manufactured from superalloys containing a gamma-prime phase and materials commonly known as the MCrAlY family of alloys. One particular problem with the gamma-prime precipitation hardenable alloys is the inability to weld or clad these alloys with like or similar alloys without encountering cracking and high production rejects. Because of the welding temperatures and stresses involved, these alloys encounter shrinkage, stress cracking and the like. Due to the difficulties in welding these specific superalloys, there is a need for a process by which gamma-prime precipitation hardened alloys can be joined consistently, without cracking, with similar or parent metal alloys.
Many years ago, sintering of discrete bodies by effecting a spark discharge between the bodies was disclosed by Inoue, in U.S. Pat. Nos. 3,250,892 and 3,241,956. This process involved disposing a mass of discrete electrically fusible particles, preferably consisting predominantly of conductive metallic bodies between a pair of electrodes which sustain a spark discharge. Since this mass tends to shrink as sintering proceeds, means were provided to maintain the electrodes in contact with the mass. To this end, the electrode means could be spring or gravity loaded to maintain the contact and, if desired, a mechanical pressure up to 100 kg./cm2 was provided, when required. The spark discharge could be terminated upon the particles being welded together, at least preliminarily, while passage of the electric current could be continued without development of the spark to weld the particles further by resistance heating. A variety of articles were made using this process: nickel and cobalt discs, bodies of copper with carbon or lead and bodies of cadmium oxide.
Intermetallic compounds, such as Fe3Al have been mechanically alloyed (xe2x80x9cMAxe2x80x9d) and then disks 10-20 mm in diameter were prepared using plasma activated sintering (xe2x80x9cPASxe2x80x9d), as described in xe2x80x9cMechanical Alloying Processing and Rapid Plasma Activated Sintering Consolidation of Nanocrystalline Iron-Aluminidesxe2x80x9d, Materials Science and Engineering, A207 (1996) pp. 153-158, by M. A. Venkataswamy et al. The PAS consolidation provided high density values in a very short time, and a very fine grain structure was maintained. PAS has also been utilized to consolidate difficult-to-sinter powders to provide AlN, Nb3Al and superconducting (Bi1.7, Pb0.3)xe2x80x94Sr2Ca2.1Cu3.1Ox ceramics with absence of significant grain growth during densification, as described in xe2x80x9cPlasma-Activated Sintering (PAS): A New Consolidation Method For Difficult-to-Sinter Materialsxe2x80x9d, Powder Metallurgy in Aerospace, Defense and Demanding Applicationsxe2x80x94Proceedings of the 3rd International Conference, published by the Metal Powder Industries Federation, (1993) pp. 309-315, by J. Hensley et al. They mention that common methods of consolidating ceramic and metallic powders include sintering, hot pressing, or hot isostatic pressing, which processes typically require long exposure (one to several hours) at high temperature, leading to grain coarsening of the microstructure and the formation of grain boundary impurities. The plasma-activated sintering (PAS) process significantly reduces the sintering times of many materials. In the PAS process, a spark plasma sintering process, the commercial powders are poured into carbon molds without additives, binders, or pre-pressing. Uniaxial pressure is applied to the powder and an external power source provides a pulsed current to activate the surface of the particles. The power supply is then switched to resistance heating for densification. To measure the sintering temperature, a thermocouple is inserted into the carbon mold and a linear gauge measures the shrinkage. They mention that the first commercial use of PAS was to manufacture Fexe2x80x94Ndxe2x80x94Coxe2x80x94B magnets around 1990.
M. Tokita, in xe2x80x9cMechanism of Spark Plasma Sinteringxe2x80x9d, Proceedings of the International Symposium on Microwave, Plasma and Thermochemical Processing of Advanced Materials, published by the Joining and Welding Research Institute, Osaka University (1997) pp. 69-76, describes SPS as being based on a high temperature plasma (spark plasma) momentarily generated in the gaps between powder materials by electrical discharge at the beginning of pulse energizing. The large current pulse energizing method generates: (1) spark plasma, (2) spark impact pressure, (3) Joule heating, and (4) an electrical field diffusion effect. The SPS process is regarded as a rapid sintering method, using self-heating action from inside the powder, similar to self-propagating high temperature synthesis (SHS) and microwave sintering. That article states that SPS systems offer many advantages over conventional systems using hot press (HP) sintering, hot isostatic pressing (HIP) or atmospheric furnaces, including ease of operation and accurate control of sintering energy as well as high sintering speed, high reproducibility, safety and reliability. The SPS process is expected to find increased use in the fabrication of functionally graded materials (FGMs), intermetallic compounds, fiber reinforced ceramics (FRC), metal matrix composites (MMC) and nanocrystalline materials, which are difficult to sinter by conventional sintering methods. Tokita explains that there was little literature on research into this process until the latter half of the 1970s. The second generation was developed from the middle of the 1980s to the early 1990s. These units were small experimental systems-Plasma Activated Sintering (PAS) with maximum sintering pressure of around 5 tons and pulse generators of up to 800 amp, used primarily for materials research. However, the third generation of this advanced technology utilizes systems with large DC pulse generators of 10 to 100 tons and 2,000 to 20,000 amp and more, which have gained a reputation as new industrial processes for synthetic processing of gradient and composite materials. Uses are listed as consolidating ceramics and cermets; polyamide, nylon and polyethylene resins; fiber/particle compounds; and magnets; intermetallics and hard metal alloys. Suitable materials for SPS processing included Fe, Ni, Cr, Sn, Ti, Q, Alxe2x80x94virtually any metal; carbides such as SiC; nitrides such as Si3N4: intermetallics such as TAl, MoSi2, NiAl, NbCo; borides; fluorides; and cermets such as Al2O3+TiC.
It is one of the main objects of this invention to provide a method of bonding sections of high temperature superalloy components, such as turbine blade sections, turbine blade tips, turbine blade root ends, and turbine vane components, by an improved sintering technique to provide high density bonding for either repair or construction of turbine components in a rapid fashion.
These and other objects of the invention are accomplished by a method of consolidating turbine component parts comprising the steps: (a) providing a turbine component part having at least two approximate opposing surfaces; (b) establishing contact pressure between the approximate opposing surfaces; (c) disposing electrodes in close proximity to the approximate opposing surfaces; (d) generating an electrical field to provide heat, and applying additional pressure on the opposing surfaces, resulting in cleaning and short-time high temperature spark plasma sintering between of the opposing surfaces; (e) reducing the electric field; (f) subjecting the two approximate opposing surfaces to additional continuing pressure; and (g) maintaining the pressure while cooling the turbine component approximate opposing surfaces. This general process could be used to mate multiple surfaces in a construction process. This process is particularly valuable in mating surfaces having different compositions and may be very useful in mating single crystal opposing surfaces. These steps, (a) through (g), are illustrated in FIG. 5, where step (axe2x80x2), shown with dotted lines, involves placing metal alloy powder between the approximate opposing surfaces. In some instances, this process might be useful for bonding ceramic opposing surfaces, optionally using ceramic particles between the surfaces.
The invention also resides in a method of repairing turbine component parts comprising the steps: (a) providing a turbine component having at least two approximate opposing surfaces; (b) inserting discrete powder, having a similar composition to at least one of the surfaces, between the at least two approximate surfaces, so that there is substantial contact pressure between the discrete particles and between the particles and the two approximate opposing surfaces of the turbine component; (c) disposing electrodes in close proximity to the opposing surfaces; (d) generating an electrical field to cause heat, and applying additional pressure between the particles, resulting in cleaning and short-time high temperature spark plasma sintering at the surface of the particles and at the approximate opposing surfaces, with deformation of the inserted particles and high speed diffusion and plastic flow of the particle material; (e) reducing the electric field between the approximate opposing surfaces; and (f) cooling the turbine component approximate opposing surfaces and the particulate material between the approximate opposing surfaces.
This process would allow repair and manufacture of complex turbine components for which casting processes are not cost effective and bonding of hybrid components in a quick operation with very short sintering times.